An MRAM device includes an array of memory cells. The typical magnetic memory cell includes a layer of magnetic film in which the magnetization is alterable and a layer of magnetic film in which the magnetization is fixed or “pinned” in a particular direction. The magnetic film having alterable magnetization may be referred to as a data storage layer or sense layer and the magnetic film which is pinned may be referred to as a reference layer.
Conductive traces (commonly referred to as word lines and bit lines) are routed across the array of memory cells. Word lines extend along rows of memory cells, and bit lines extend along columns of memory cells. Because the word lines and bit lines operate in combination to switch the orientation of magnetization of the selected memory cell (i.e., to write the memory cell) the word lines and bit lines can be collectively referred to as write lines. Additionally, the write lines can also be used to read the logic values stored in the memory cell.
Located at each intersection of a word line and a bit line is a memory cell. Each memory cell stores a bit of information as an orientation of a magnetization. External magnetic fields are applied to flip the orientation of magnetization in the data storage layer with respect to the orientation of magnetization in the reference layer, depending on the desired logic state (i.e., “1” or “0”).
The orientation of magnetization of each memory cell will assume one of two stable orientations at any given time. These two stable orientations represent logic values of “1” and “0”. The orientation of magnetization of a selected memory cell may be changed by supplying current to a word line and a bit line which intersect at the selected memory cell. The currents create magnetic fields that, when combined, can switch the orientation of magnetization (and thus the logic value) of the selected memory cell. Since no electric power is needed to maintain the memory state of the device, MRAM's are non-volatile.
A selected magnetic memory cell is usually written by applying electrical currents to the particular word and bit lines that intersect the selected magnetic memory cell. The electrical currents create a corresponding magnetic field (a “write field”) about the energized word and bit lines. Preferably, only the selected magnetic memory cell receives both the word and bit line write fields. Other memory cells coupled to the particular word line preferably receive only the word line write field. Other magnetic memory cells coupled to the bit line preferably receive only the bit line write field.
The magnitudes of the word and bit line write fields are usually selected to be high enough so that the chosen magnetic memory cell switches its logic state when subjected to both fields, but low enough so that the other magnetic memory cells which are subject only to a single write field (from either the word line or the bit line) do not switch. The undesirable switching of a magnetic memory cell that receives only one write field is commonly referred to as “half-select” switching.
In MRAM designs having memory cells with shapes other than a toroidal shape (e.g., rectangular shapes), the magnetic moment in the memory cell is linearly oriented. The primary problem of linear magnetization orientations is the stray field (outside of the magnetic elements of the memory cell) and the demagnetization field (within the magnetic elements of the memory cell) generated from the magnetic poles formed at the end of the memory cell. If the ends of the magnetic elements are flat, the strong demagnetization field can cause the formation of complicated edge domains and thereby cause the switching threshold of the magnetic moment to fluctuate uncontrollably. Therefore, in practice, the ends of the memory cells are preferably tapered into sharp tips to eliminate or reduce edge domains.
The preference for sharp ends in the memory cells may force the size of the memory cell to be much larger than the critical dimension of the fabrication technology used to make the device. Shape variations of the tapered ends from element to element (such as due to a fabrication process variation) could yield variations of the switching field, thereby degrading the capability of the memory cell. In addition, even tapered ends may generate a stray magnetic field which could interfere with adjacent memory cells in an array, thereby limiting the packing density of the memory cells.
The memory element in the MRAM device of the present invention is a toroid-shaped stack having a sense layer and a pinned reference layer separated by an insulator. Toroid-shaped memory elements are previously known, and offer several advantages. The circular magnetization mode of the toriodal shaped memory cell provides a stable magnetic configuration which produces no stray fields and no demagnetization field. Thus, the toroid-shaped memory cell has an advantage in that it may be placed in higher densities in an array, and can be manufactured in sized closer to the critical dimensions allowed by the fabrication technology.
The toroidal shape of the memory cell results in a circular orientation of the magnetization, either clockwise or counterclockwise. The magnetization orientation of the sense layer of the memory cell may be switched either from clockwise to counter-clockwise or vice versa. A current pulse in a conductor extending over or under the memory cell generates a radial magnetic field, rotating the magnetization in the sense layer in the radial direction. A current pulse in a conductor extending through the axial opening of the toroid-shaped sense layer generates an angular (or circular) magnetic field, switching the magnetization into the new circular orientation. When the magnetization orientations of the pinned layer and the sense layer of the memory cell have the same directional orientation, the resistance of the memory cell is low (corresponding, for example, to a logic “1”). When the pinned layer and the sense layer have magnetization orientations in opposite directions, the resistance of memory cell is high (corresponding, for example, to a logic “0”).
Although toroid-like memory cells provide advantages over linearly oriented memory cells, as discussed above, is still desirable to reduce the power consumption in the MRAM device using toroid-like memory cells by reducing the magnitude of the write currents necessary to rotate the orientation of magnetization of the data storage layer during write operations to the memory cell. For example, reduced power can result in a reduction in the waste heat generated by electronic devices incorporating the MRAM device. Moreover, for portable devices, is desirable to reduce power consumption to extend battery life.